Leap Years, Leap Centuries

Century years, such as 1900 2000, are leap years if they are evenly divisible by 400. The

basic rules for calculating leap years are:

  1. Years divisible by 4 are leap years, but

  2. Years divisible by 100 are not leap years, but

  3. Years divisible by 400 are leap years.

The need for leap years is due to the fact that the actual length of a year is 365.242 days,

not 365, as is commonly stated. To account for this, an extra day is added as February 29th

on years that are evenly divisible by 4 (eg. 1992).

Since the year is slightly less than 365.25 days long, adding an extra day every 4 years

results in about 3 extra days being added over a 400-year period. For this reason, 1 out of

every 4 century years also needs to be a leap year.

Using this arrangement a year has 365.2425 days on the average.